The present invention relates generally to blood pumps for a living body, and, more specifically, to bearings therein.
U.S. Pat. No. 5,924,975-Goldowsky discloses a blood pump configured as left ventricular assist device (LVAD) for pumping blood in a living body. The pump includes a reciprocating piston inside a housing bore which is bathed entirely in the blood being pumped. The outer surface of the piston defines a journal cooperating with the bore of the housing to effect a hydrodynamic journal bearing for supporting the piston in the housing as it reciprocates.
The main function of the piston is to pump blood through a living body, yet this must be accomplished without damaging that blood. Accordingly, the various features of the piston for effecting its reciprocation must be designed for minimizing blood damage during operation and continuously flushing blood past the piston for preventing stagnation thereof and minimizing blood damage.
The piston is self supported during operation by developing hydrodynamic pressure in the blood within the small radial gap between the piston journal and housing bore defining the journal bearing. The gap must be sufficiently small, on the order of about 1 or 2 mils, for developing hydrodynamic pressure forces, yet cannot be too small which would increase shear forces in the blood and corresponding damage thereto. Furthermore, the bearing gap must be sufficiently sized to permit suitable flushing or wash-out of the blood therethrough for avoiding blood thrombosis due to insufficient washout flowrate.
Correspondingly, an excessive bearing gap greater than a few mils will destroy the hydrodynamic pressure capability of the journal bearing rendering it ineffective.
A nominal bearing gap is sufficiently small for developing hydrodynamic pressure forces during operation yet nevertheless permits a relatively small amount of piston tilting or cocking during transient or steady-state operation of the piston as it reciprocates during operation. Piston tilting locally reduces the bearing gap at the opposite ends of the piston, and must therefore be minimized.
Resistance to piston tilting may be provided by increasing the axial length of the piston itself, yet this in turn increases the overall length of the blood pump which is undesirable for obtaining a blood pump of compact size for being surgically implanted in a patient.
Accordingly, it is desired to improve the tilt resistance or stiffness of the journal bearing supported piston in the blood pump without increasing its overall length and without increasing blood damage during operation.
A hydrodynamic blood bearing includes a piston disposed coaxially in a housing bore. The piston includes a cylindrical outer surface journal spaced from the bore to define a first radial gap for receiving blood as a bearing fluid for hydrodynamically supporting the piston. The piston includes two sets of bearing pads axially spaced apart from each other, with each set including circumferentially spaced apart pads for increasing tilt stiffness of the bearing defined by the journal.